Ozone Layer Destruction

Here is a short summary of the anthropogenic causes of stratospheric ozone layer destruction

Stratospheric ozone, the ozone layer, absorbs much (but not all) of the dangerous high energy ultraviolet light from the sun. Listed above are some of the serious hazards or problems associated with exposure to ultraviolet light. Human activities add substances to the atmosphere that can potentially reduce ozone concentration in the ozone layer (which would result in increased exposure to UV light at the ground).

The first set of reactions above involve nitric oxide, NO. First, NO reacts with O3 to form NO2 and O2 (ordinary molecular oxygen). Then notice the NO2 reacts with an oxygen atom (which might otherwise react with O2 to make O3) to form NO again and O2. The NO is available again to react with and destroy another ozone molecule.

At one time many countries were considering building fleets of supersonic aircraft that would fly into the stratosphere. The plans were scrapped partly due to concern that the NO emissions from these aircraft would damage the ozone layer.

The main threat now comes from chlorofluorocarbons (CFCs). CFCs were at one time thought to be an ideal industrial chemical and had a variety of uses. CFCs are unreactive, non toxic, and stable. Once they get into the atmosphere they remain there a long time, as much as 100 years. The reactions involving CFCs are shown on the next figure. CFCs released at ground level [lower left corner in the figure above] remain in the atmosphere long enough that they can eventually make their way up into the stratophere. UV light can then break chlorine atoms off the CFC molecule [a]. The resulting "free chlorine" can react with and destroy ozone. This is shown in (b) above. Note how the chlorine atoms reappears at the end of the two step reaction. A single chlorine atom can destroy 100,000 ozone molecules. There are ways of removing chlorine from the atmosphere. A couple of these so called "interference reactions" are shown in (c) above. The reaction products, reservoir molecules (because they store chlorine), might serve as condensation nuclei for cloud droplets (the small water drops that clouds are composed of) or might dissolve in the water in clouds. In either event the chlorine containing chemical is removed from the atmosphere by falling precipitation. Clouds are probably the most effective way of cleaning the atmosphere.

The ozone hole that forms above the S. Pole every year in late September-early October was one of the first real indications that CFCs could react with and destroy stratospheric ozone. The hole is not really a hole in the ozone layer, just a temporary thinning of the ozone layer above the S. Pole and the continent of Antarctica. The ozone concentration decreases to perhaps 30% of its normal value. It is unusual to find clouds in the stratosphere. It gets very cold above the S. Pole in the winter and polar stratospheric clouds do sometimes form (they are made from water and other materials). This together with an unusual wind pattern above the S. Pole in the winter are thought to create the ozone hole when the sun returns in the spring.

The ozone destruction reactions are shown in purple above. Cl reacts with O3 to make ClO. This reacts with O to produce Cl and O2. The Cl is now available to react again with other ozone molecules. In green are "interference" reactions. ClO reacts with NO2 to make ClNO3. The Cl in this "reservoir" molecule can't react with any more ozone.

Now what happens above the S. Pole in the winter is that the reservoir molecules react on the surfaces of the polar stratospheric cloud particles to make some kind of new compound. This reaction is shown in orange above. The new compound HOCl accumulates in the air during the winter. When the sun reappears in the spring, the UV light splits off all the Cl molecules which react with ozone. A lot of chlorine suddenly becomes available and the ozone concentration takes a nosedive.